The good news is, it only raises this error if it was successful in creating the .fmb file.

The easiest way to find out what's wrong is to open up the .FMB file and try to compile it from Forms Builder, then see what you have to do to make it work from there (change syntax, etc.). Once you have that figured out, go back to Designer and fix your definition of your client side function and/or your unbound item.

If you're really good, you can just open up the .err file that was created, see the compile error, and fix it in Designer, but that almost never works for me.